The present invention generally relates to a ceramic/metallic assembly including a ceramic layer and a metallic layer bonded thereto, and an electrostatic chuck using such an assembly. The present invention more particularly relates to a ceramic/metallic assembly, and an electrostatic chuck using such an assembly which are characterized by the material of the ceramic layer and the bonding arrangement between the ceramic and metallic layers.
Layered ceramic/metallic assemblies have been used in various fields such as electrostatic chucks as disclosed in Japanese patent laid open publication No. 10-41377 filed by the assignee of the present application, and various bonding techniques have been proposed for the manufacture of such assemblies.
In recent years, larger and larger ceramic/metallic assemblies are being demanded particularly for use in electrostatic chucks for handling large semiconductor wafers. As the size of the ceramic/metallic assembly gets larger, the bonding strength is required to be correspondingly higher. When a ceramic/metallic assembly is subjected to large changes in temperature, the resulting thermal stress tends to cause a damage to the bonding layer or generate cracks in the ceramic layer. Such tendencies are progressively pronounced as the size of the assembly increases.
Additionally, the resistance of the assembly against corrosion is important because corrosion could cause emission of sintering additives in the ceramic layer, and such an emission pollutes the environment in which the assembly is used. The assembly is often used in a vacuum chamber virtually free from any contaminants, and is subjected to high energy beams and corrosive substances.
In view of such problems of the prior art, a primary object of the present invention is to provide a ceramic/metallic assembly which is mechanically strong, and highly durable in use.
A second object of the present invention is to provide a ceramic/metallic assembly which would not emit a pollutant even when exposed to high energy beams and corrosive substances.
A third object of the present invention is to provide an electrostatic chuck using such a ceramic/metallic assembly.
According to the present invention, such objects can be accomplished by providing a ceramic/metallic assembly, comprising: a ceramic layer essentially consisting of a magnesia ceramic material; and a metallic layer essentially consisting of a metallic material having a thermal expansion coefficient similar to that of the ceramic layer, the ceramic layer and the metallic layer being directly bonded to each other.
Magnesia ceramic material has a greater thermal expansion coefficient than other ceramic materials such as alumina and aluminum nitride as shown in Table 1.
Therefore, magnesia ceramic material is more suited to be directly bonded, typically by brazing, to a wide range of metallic materials than other ceramic materials, and allows a relatively large ceramic/metallic assembly to be produced without risking the durability of the assembly. Furthermore, a ceramic/metallic assembly using magnesia ceramic material is capable of withstanding more severe thermal stress than those using other ceramic materials such as alumina and aluminum nitride.
Also, magnesia may be sintered into a dense ceramic material without using any sintering additives. In other words, it is possible to obtain magnesia ceramic material of a high purity of 99.9% or better. Therefore, magnesia ceramic material would not pollute the environment in which the assembly is used. If desired, other ceramic materials such as silicon dioxide, titanium dioxide, titanium carbide and alumina may be added to the magnesia ceramic to obtain a ceramic material having a desired dielectric property and thermal conductivity.
It is preferable to select the material for the metallic layer, which is to be bonded to the ceramic layer, from those having thermal expansion coefficients similar to that of the magnesia ceramic material such as ferritic stainless steel. In such a case, the thermal expansion coefficients of the ceramic material and the metallic material typically range from 12xc3x9710xe2x88x926 (1/xc2x0 C.) to 15xc3x9710xe2x88x926 (1/xc2x0 C.).
When metallic materials such as aluminum and aluminum alloys having higher thermal expansion coefficients are desired to be used, it is preferable to place one or more damper layers having intermediate thermal expansion coefficients between the ceramic layer and the metallic layer. Typically, the intermediate layer essentially consists of a ceramic/metallic compound material. For details of such intermediate layers, reference should be made to the aforementioned Japanese patent laid open publication. In such a case, typically, the thermal expansion coefficient of the ceramic material is from 12xc3x9710xe2x88x926 (1/xc2x0 C.) to 15xc3x9710xe2x88x926 (1/xc2x0 C.), the thermal expansion coefficient of the metallic material, typically consisting of aluminum or an aluminum alloy, is greater than 15xc3x9710xe2x88x926 (1/xc2x0 C.), and the thermal expansion coefficient of the intermediate layer is from 12xc3x9710xe2x88x926 (1/xc2x0 C.) to 15xc3x9710xe2x88x926 (1/xc2x0 C.).
Such ceramic/metallic assemblies can be most advantageously used in a semiconductor wafer retaining fixture as a supporting base for a semiconductor wafer. If the ceramic layer incorporates an electrostatic electrode arrangement therein, the fixture can be used as an electrostatic chuck. If the metallic layer incorporates a heating/cooling arrangement therein, the fixture can be used for controlling the temperature of the wafer retained thereby. However, as can be readily appreciated by a person skilled in the art, the ceramic/metallic assembly can also be used in any other applications where thermal stress may be of concern.